1. The Field of the Invention
This invention concerns improvements in and relating to verification and/or calibration, particularly, but not exclusively, to verification and/or calibration in relation to characterisation of spent nuclear fuel.
2. The Relevant Technology
Nuclear power generation involves, the provision of fuel assemblies containing enriched fuel to a reactor; the residence of that fuel assembly in the reactor, during which time the fissile material is consumed; and the removal of the fuel assembly after a period of time for subsequent handling. The subsequent handling may include reprocessing, to recover fissile material for subsequent use, and/or storage and/or transportation of the fuel.
In such subsequent tasks it is desirable for a number of reasons to have information on the characteristics of the fuel in the fuel assemblies.
A variety of instruments for monitoring such characteristics to varying degrees are known, including high resolution gamma spectrometry (HRGS), low resolution gamma spectrometry (LRGS) and passive neutron counting.
A variety of actual emission forms can be monitored to calculate the burnup of the fuel, either according to the level of that emission form or the interrelationship of two or more emission forms to one another.
A frequently monitored emission is based on curium emissions from the spent fuel. To achieve a detailed measurement of burnup, however, this measurement requires information on the original enrichment of the fuel as the curium based count is not only based on the burnup of the fuel, but also its starting enrichment.
The present invention aims to provide a technique for accurately verifying the consistency of operation of an instrument for measuring burnup. The invention also aims to allow the claimed history for the fuel to be independently verified by comparing a model based evaluation of the claimed fuel history against the independent measured value. The provision of improved sources for calibration and/or modelling purposes and/or of calibration and/or verification test rigs is also an aim of the present invention.
According to a first aspect of the invention we provide a method of investigating the response of an instrument to gamma emissions using a source package, the method comprising:
providing a source package on one side of a collimator including an aperture and a detector on the other side of the collimator relative to the source package, the collimator and detector positions defining an operating axis for the instrument;
moving the position of the source package relative to the collimator, with the response of the detector being measured for two or more of those different positions of the source package;
an aperture in the collimator having a first dimension measured in a first direction perpendicular to the operating axis;
the source package comprises one or more individual gamma emitting sources provided in a container, in use the individual source(s) having an overall extent in the first direction, the overall extent, during at least 10 seconds of use, being two or more times the first dimension of the collimator in that first direction.
The operating axis may be thought of as the centre line of the field of view defined by the collimator or collimators and detector and/or an axis generally corresponding to the direction of travel of gamma emissions detected by the instrument.
According to a second aspect of the invention, we provide a source of gamma emissions, the source comprising one or more individual gamma emitting sources provided in a container, the individual source(s) have an overall extent in a first direction, that extent being greater than 10 mm.
The first and/or second aspect of the invention may include any one or more of the following features, options and possibilities.
Preferably the overall extent of the individual source(s) in the first direction is less than 50 mm, more preferably less than 30 mm.
The individual sources have an overall extent in a second direction, for at least 10 seconds in use, ideally perpendicular to the first direction. The second extent may be less than 20 mm and preferably less than 10 mm. Preferably the extent in the second direction is at least 1.5 mm and more preferably at least 2 mm. The second extent is preferably the same in all directions perpendicular to the first.
The first and/or second extent of the source package during the at least 10 seconds of use may be provided by the actual dimension of the source or sources in that direction. The extent of the source package during the at least 10 seconds of use may be provided by oscillating a source or sources through that extent, the source or sources having an actual dimension in that direction less that the extent. Preferably a frequency of oscillation of greater than 10 Hz is provided.
A plurality of individual sources may be provided in the container. The plurality of sources maybe a number greater than 5, greater than 10 or even 15 or more.
The gamma emissions from the individual sources are preferably of the same energy and/or same energies and/or same range of energies as are used by the instrument in its measurements.
Preferably each of the individual gamma sources is of the same type. Preferably each of the individual gamma sources emits the same gamma emissions, in terms of their energy energies or range of energies. The individual sources may be or contain 137Cs.
Preferably the level of emissions from each of the individual sources is substantially the same, for instance within 10% of one another. Preferably the level of emissions for an individual sources is substantially consistent from all directions, at least in all directions perpendicular to the axis on which the sources are aligned, for instance the second direction. Preferably the quantity of gamma emitter, ideally the quantity of the selected gamma emitter, such as 137Cs, will be sufficient to give a nominal equivalent activity of at least 5 mCi and/or a nominal equivalent activity of at most 40 mCi.
Preferably each of the individual gamma sources is substantially the same shape. Preferably the individual sources are cylindrical.
The individual source or sources may be provided with an end face at one or both ends of the extent in the first direction. One or both of the end faces of one or more, ideally all, the sources are preferably planar and/or parallel to one another. Preferably at least the outside end face of the individual source defining one end of the overall extent and the outside end face of the individual source defining the other end of the overall extent are parallel to one another, ideally, they are also perpendicular to the axis on which the sources are aligned. The axis is preferably the first direction. One, preferably both, of the end faces of one or more of the individual source(s) may be perpendicular to the axis of the source and/or the cylindrical surface thereof.
The individual source or sources may be provided with an effective side edge. The effective side edge may be defined by a face, but is more preferably the extent of the cylindrical surface of the source when viewed in from the side, for instance, perpendicular to the axis of the cylinder. One, preferably both of the effective side edges of one or more, ideally all, the individual sources are preferably linear and/or parallel to one another. Ideally, the effective side edges are aligned with one another, at least in the second direction. Preferably the effective side edges are parallel to the axis on which the individual sources are aligned. The axis is preferably the first direction.
The sources are preferably aligned on a common axis with one another.
The individual sources, for instance as cylinders, may be between 0.5 mm and 1.5 mm in length. The individual sources may be between 1 mm and 4 mm in diameter.
The container may be of stainless steel. Preferably the container is cylindrical. The end faces of the container are preferably parallel to one another. One, preferably both, of the end faces of the cylinder may be perpendicular to the axis of the cylinder and/or the cylindrical surface.
The container, for instance cylinder, may be between 10 mm and 30 mm in length. The cylinder may be between 5 mm and 25 mm in outside diameter.
The container may provide a bore in which the individual sources are received. Preferably the minimum internal cross-section measurement of the bore generally corresponds, for instance the same as or plus up to 2%, the maximum external cross-section measurement of the individual sources. The individual sources may have a cross-section which corresponds to the cross-section of the bore. The bore is preferably closed once the sources have been introduced. The closure may be of stainless steel.
Preferably the container provides a constant thickness of material between the individual sources and the outside of the container, for instance +/xe2x88x9215%, more preferably +/xe2x88x925%. The thickness of the material may be 2 mm and 10 mm.
Preferably the container length and/or overall extent of the sources is at least twice the smaller of the height or width of the aperture in the collimator of the instrument in question. The height and width are generally those dimension substantially perpendicular to the direction in which gamma emissions pass through the collimator.
The position of the individual sources within the container is preferably fixed relative to the container. Preferably the position is fixed in the first direction and/or the second direction. The position may be fixed by cooperation of the sources with the container and/or with one another and/or packaging material provided within the container.
The source may be used in a method of checking instrument performance. The method may include determining the instrument""s response to the source package, then conducting one or more investigative measurements on unknown samples and determining the instrument""s response to the source package after those investigative measurements. A consistent response may be taken as indicative of consistent instrument performance during the measurements.
The source package and/or method of investigating an instrument""s response may be used in a method of verifying information concerning materials. The method can comprise: providing an instrument, using the instrument to measure emissions from one or more radioactive sources within the material, the instrument producing signals upon detecting the emissions, the signals being processed to give a measured indication of a characteristic of the material
The method can further comprise obtaining information about the history of the material, inputting at least some of that information to a modelling process, the modelling process generating a representation of the emissions from material having the inputted information, the representation of the emissions being applied to a further modelling process for the instrument, the further modelling process, at least in part using the source package and/or method of investigating an instrument""s response, and producing a modelled indication of the characteristic of the material.
The method can yet further comprise comparing the measured indication of the characteristic and the modelled indication of the characteristic to verify the information inputted.
The source package and/or method of investigating an instrument""s response may be used to model the effects of the field of view of the detector and/or variation in detection within the field of view.
The source during modelling and/or during checking of instrument performance may be moved relative to the field of view of the instrument. The source may be moved across the field of view from one side to the other. The one side may be above the other or vice versa. The one side may be to one side and/or the other side of the other horizontally.
The information to be verified may be one or more characteristics of the materials past or a characteristic derived therefrom, such as the burnup of the material and particularly the claimed burnup.
It is preferred that the emissions are measured by detecting gamma emissions using a collimated detector. The detected emissions are preferably converted to electrical signals and preferably fed to signal processing means. The signals preferably generate or correspond to a count rate, ideally for one emission form. Preferably the measured count rate is compared with a known relationship to give the measured indication of the characteristic.
The known relationship may be a linear relationship between count rate and the indication. The known relationship is preferably independent of the level of fissile material in the material initially.
The measured indication may be expressed as a burnup value.
The information about the history of the material may be obtained from the utility operator whose reactor the material has come from and/or the manufacturer of the material and/or the operator of the facility the material is stored in and/or the regulatory authority.
The modelling process may generate one or more factors providing the representation of the emissions arising from material having that history, preferably the emissions represented are the same emissions measured by the instrument. The representation may relate to the level of emissions arising from material having that history, for instance the escape probability for those emissions. The modelling process preferably produces an escape probability for 137Cs emissions, ideally 662 keV emissions.
The further modelling process preferably models the effects of the instrument on the representation of the emissions, for instance on the emissions themselves and most preferably the count rate arising from those emissions. The further modelling process may account for the level of emissions entering the instrument and/or the effect of the collimator and/or the effect of the detector and/or the effect of the signal processing means on the emissions modelled as being detected. The model may incorporate information on the response of the instrument to emissions, the information being generated by measuring the instrument""s response to a source according to the present invention. The model may generate a count rate for the emissions and preferably generates an indication of the same characteristic as the measured indication of characteristic, based on the inputted history information. The modelled characteristic may be burnup.
The comparison may be a comparison of the measured characteristic""s value and the modelled characteristic""s value. The comparison may determine whether the values match and/or whether the values do not match. A match may be deemed to be a modelled value within a predetermined range of the measured value or vice versa.
A match between measured and modelled values may be taken as confirming the historical information for the material as being correct. A non-match between measured and modelled values may be taken as indicating a deviation in the actual history of the material from the inputted historical information. The deviation may occur in one or more of the components of the historical information.
According to a third aspect of the invention, we provide apparatus for checking instrument performance and/or for modelling instrument performance. The instrument comprises a detector to measure emissions from one or more radioactive sources within a source package, the detector producing signals upon detecting the emissions. The signals are fed to signal processing means, the signal processing means providing a measured indication of a characteristic of the source package, for instance a count.
The apparatus also includes a mount for holding the source and support means for the mount. The mount is moveable relative to the instrument in a first, preferably horizontal plane and/or in a second, preferably vertical plane and/or combinations thereof.
Preferably the support frame for the mount is mounted on the instrument in use, for instance on the collimator and/or casing thereof.
Preferably the movement of the source is affected by a motor, most preferably with the motor providing force to the mount holding the source so as to move the mount along a pre-defined path. Preferably the motor and/or mount for the source are computer controlled. This offers reproducible positioning of the source.
The third aspect of the invention may include any of the features, options or possibilities set out n the first aspect of the invention.
The first and/or second and/or third aspects of the invention may include any of the following features, options or possibilities.
The emissions may be neutrons, but are preferably gamma rays. The monitored emissions may be of an energy specific for one or more of the sources. The emissions may have an energy of between 100 and 2500 keV, for instance 630 and 690 keV, and more particularly, 662 keV.
The radioactive sources may be one or more isotopes emitting gamma rays and/or neutrons. Preferably the radioactive sources are gamma ray emitters. The radioactive sources may be fissile or fissionable isotopes present in new or recycled fuel, but are more preferably one or more fission products. It is particularly preferred that the radioactive source be a direct fission product of uranium and/plutonium, and more preferably a direct fission product which is yielded substantially equally from uranium and plutonium. The radioactive source may be 137Cs.
The material may be fresh or recycled nuclear fuel, but is more preferably nuclear fuel which has passed through a nuclear reactor. The instrument is particularly suited to monitoring spent nuclear fuel in the fuel rods and/or fuel rod assemblies in which the fuel was in the nuclear reactor.
The instrument may comprise an elongate element with the detecting location and/or collimator and/or material presentation location, and preferably all three, provided towards one end of that elongate element. The elongate element may be a hollow tube. The detector and/or detector assembly may be contained in the elongate element. The elongate element interior is preferably sealed against the environment surrounding the elongate element, particularly the further elongate element. The environment may be water. The elongate element interior is preferably open to the air, for instance above the water level of the environment. It is preferred that the elongate element provides a low radiation level environment within the elongate element in the vicinity of the detector and/or detector location. That low radiation level environment is preferably provided by the instrument within a high level radiation environment, for instance cooling water containing fuel rods.
In use, the elongate element is preferably introduced into a cooling pond or other liquid containing vessel in which the material to be monitored is present. Preferably the elongate element is substantially vertically aligned when deployed. The elongate element may be attached directly to the side of the cooling pond or vessel and/or indirectly mounted thereon.
The signal processing means may be provided separate from the elongate element and is preferably provided at least 5 m from the cooling pond or vessel containing the elongate element.
The signal processing means may convert the detector generated signals into a detector count and/or gamma spectrum. The signal processing means may convert the detector generated signals into a measurement of nuclear fuel burnup and/or cooling time and/or axial burnup profile. The signal processing means may be used to calculate a burnup credit, for instance via the calculation of burnup, from the detector generated signals. One or more of these processing steps may be conducted via the calculation of a count rate for the emissions based on the detector generator signals.
The signal processing means may comprise a computer, micro-processor or the like. The signal processing means may include data storage and/or manipulation and/or presentation capability.
The detector may be linked to the signal processing means via an electrical connection. The electrical connection may pass through the inside of the elongate element. Preferably the connection from the detector to the signal processing means passes into the detector assembly through the top of the detector assembly, that is the end opposing the supporting location engaging portion. The link to the detector is preferably electrically and/or electromagnetically isolated from the detector assembly.
Preferably the collimator is mounted inside a further element. Preferably the further element is elongate. The further element may extend perpendicularly from the elongate element. Most preferably, the further element extends perpendicularly from the elongate element. The further element is preferably provided at one end of the elongate element, i.e. within 5% of the overall length of the end of the elongate element.
The detector is preferably a high resolution gamma detector, for instance a high purity germanium detector. The detector may include a cryostat and liquid nitrogen coolant.
Preferably the collimator is elongate. The axis of the collimator preferably coincides with the axis of the further element. Preferably the axis of the collimator is perpendicular to the axis of the elongate element.
The collimator may be provided with one or more apertures. Preferably, the apertures define the field of view of the material presentation location from the detecting location and/or from the detector.
The material presentation location may be anywhere within the field of view of the detector, but is preferably adjacent to the end of the further element or in contact therewith. The end of the further element being the end distal from the detecting location. It is particularly preferred that at least part of the material be in contact with the further element during monitoring.
Preferably the detecting location, and collimator are provided within an enclosure sealed against liquids. A liquid impermeable, gamma ray transparent material is preferably provided between the material presentation location and the collimator.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.